Catalpol also called (1aS,1bS,2S,5aR,6S,6aS)-1a,1b,2,5a,6,6a-Hexahydro-6-hydroxy-1a-(hydroxymethyl)oxir is a natural iridoid found in the herb of Rehmannia glutinosa (Gaert.) Libosch. ex Fisch. et Mey, it is a kind of enzyme inhibitors, has antihistaminic activity. Catalpol also exhibits the activity of anti-inflammatory.